1. Field of the Invention
The present invention relates to isolated polypeptides having acetylxylan esterase activity and isolated polynucleotides encoding the polypeptides. The invention also relates to nucleic acid constructs, vectors, and host cells comprising the polynucleotides as well as methods of producing and using the polypeptides.
2. Description of the Related Art
Plant cell wall polysaccharides constitute 90% of the plant cell wall and can be divided into three groups: cellulose, hemicellulose, and pectin. Cellulose represents the major constituent of call wall polysaccharides. Hemicelluloses are the second most abundant constituent of plant cell walls. The major hemicellulose polymer is xylan. The structure of xylans found in cell walls of plants can differ significantly depending on their origin, but they always contain a beta-1,4-linked D-xylose backbone. The beta-1,4-linked D-xylose backbone can be substituted by various side groups, such as L-arabinosyl, D-galactosyl, acetyl, feruloyl, p-coumaroyl, and glucuronyl residues.
The biodegradation of the xylan backbone depends on two classes of enzymes: endoxylanases and beta-xylosidases. Endoxylanases (EC 3.2.1.8) cleave the xylan backbone into smaller oligosaccharides, which can be further degraded to xylose by beta-xylosidases (EC 3.2.1.37). Other enzymes involved in the degradation of xylan include, for example, acetylxylan esterase, arabinase, alpha-glucuronidase, ferulic acid esterase, and p-coumaric acid esterase.
Acetylxylan esterase (EC 3.1.1.6) removes the O-acetyl groups from positions 2 and/or 3 on the beta-D-xylopyranosyl residues of acetylxylan. Acetylxylan plays an important role in the hydrolysis of xylan because the acetyl side groups can interfere sterically with the approach of enzymes that cleave the backbone. Removal of the acetyl side groups facilitates the action of endoxylanases. A classification system for carbohydrate esterases, based on sequence similarity, has led to the definition of 13 families, seven of which contain acetylxylan esterases (Henrissat B., 1991, Biochem. J. 280: 309-316, and Henrissat and Bairoch, 1996, Biochem. J. 316: 695-696).
Margolles-Clark et al., 1996, Eur. J. Biochem. 237: 553-560, disclose an acetylxylan esterase from Trichoderma reesei. Sundberg and Poutanen, 1991, Biotechnol. Appl. Biochem. 13: 1-11, disclose the purification and properties of two acetylxylan esterases of Trichoderma reesei. WO 2005/001036 discloses an acetylxylan esterase gene from Trichoderma reesei. U.S. Pat. No. 5,681,732 discloses an acetylxylan esterase gene from Aspergillus niger. U.S. Pat. No. 5,763,260 discloses methods for altering the properties of acetylated xylan.
The present invention relates to polypeptides having acetylxylan esterase activity and polynucleotides encoding the polypeptides.